The basic principle of such a device includes that a wave conductor made from an electrically conductive and also magnetizable material extends in the measurement direction, thus the longitudinal direction of the device.
A magnet is connected to the component, whose position in longitudinal direction shall be measured or monitored, and is being moved touch free by the component in longitudinal direction along the wave conductor, however, at a distance which is sufficiently small.
A current impulse imparted into the wave conductor causes a mechanical-elastic density wave through the interaction with the magnet, which density wave runs from the position of the magnet along the wave conductor, and is detected with respect to its runtime by the processing electronics, which is typically disposed at one end of the wave conductor, and the longitudinal position of the magnet relative to the wave conductor and thus of the component to be monitored are known from the detection.
Since such travel distance measurement devices are also often being used in production equipment, they have to comply with numerous requirements, like protection of the measurement device against mechanical damages and contamination, in particular against penetration of moisture into the processing electronics, maintaining the original condition of the wave conductor, shielding the electromagnetic radiation of the processing electronics in outward and inward direction, ease of assembly and maintenance of the measurement device.
On the one hand, it is already known in this respect to receive the wave conductor in a supporting, however, not excessively damping encasement, and to handle it as a wave conductor unit in this configuration.
On the other hand, it is already known to dispose the wave conductor or said wave conductor unit in a protected manner in the interior of a circumferentially closed hollow protective profile, which can be produced as an extruded profile in a cost effective manner.
Thus, for different applications, different shapes of protective profiles are required, e.g. as a function of                how stable the protective profile has to be with respect to bending in transversal direction;        if the magnet has to be, or can be supported form locked or freely moving along the protective profile in order to influence the longitudinal sensor;        the protective profile itself has to be receivable in a particularly circular inner contour, in particular in another protective tube;        how much cross sectional space is provided for mounting a protective profile, in particular in which ratios (width versus height).        
Thus it is the most common boundary condition that only a very small height is available for disposing the sensor unit and thus of the protective profile, typically in an order of magnitude of only 12 mm at the most, while the available width is significantly greater.